1. Field of the Invention
This invention relates to photoelectrically controlled apparatus for following a line or edge so as to control the movement of one or more associated cutting tools positioned over a steel plate or other workpiece to be cut.
The line or edge is usually provided by a drawing fixed to a horizontal table positioned below the follower, although it may be the actual seam to be welded, or an offset line on the workpiece. The follower may be mounted on a boom carrying tools in the form of one or more gas-fuelled cutting torches, in which case there is a 1:1 ratio between the drawing and the path traced out by the or each cutting torch. Alternatively, the drawing may be on a reduced scale of say 1:10, and the follower is separate from the torch(es), which is or are constrained to move at 10 times the speed at which the follower moves over the drawing, so that the path followed by the or each torch is 10 times the size of the drawing, to give so-called `ratio-tracing`.
2. Description of the Prior Art
In photoelectrically-controlled line followers, with which the present invention is concerned, an optical image of the line being followed is oscillated across a photoelectric cell to produce an electric signal which is modulated in accordance with the variation in the amount of light reflected from the drawing as the point-of-scan of the follower oscillates in a plane substantially perpendicular to the axis of the line. As the follower is driven along the line; by the selective energisation of two `coordinate` motors controlled by the signal from the photoelectric cell, the point-of-scan traces out a substantially sinusoidal curve. The signal generated when the follower is scanning a white surface is compared with the signal generated from a black surface to give an `error signal` indicative of the relative dispositions of the center of the scan from either the centre or a selected edge of the line being followed. This error signal is then used to control the drive motors so as to reduce the error signal to substantially zero. Thus the line follower is servo-controlled.
In practice no surface is perfectly white, i.e. it does not reflect all the incident light, but absorbs some of it; nor is any surface perfectly black, but always reflects some incident light. By `white` in this specification and claims is meant any pale surface which reflects more than 1.5 times the amount of light reflected from a dark (`black`) surface from which it is separated by a relatively well-defined boundary. The word `light` in this specification and claims means radiation having a wavelength in that part of the electromagnetic spectrum to which the photocell is sensitive. In the case of the so-called `dye-line` prints, normal visible light is reflected with a significant contrast from the white and black areas of the print. However, photoelectric cells are usually more efficient at converting to electrical signals radiation in the infra-red part of the spectrum than in the visible part. It so happens that, in most normal dye-line prints, in the infra-red range the reflectance of the white and balck areas of the print is almost identical so that, as far as the photoelectric cell is concerned, the lines of the print are invisible: they present a less-than-acceptable contrast with the background of the print.